I recently travelled to southern British Columbia, Canada. I visited
the Vancouver area as well as areas hundreds of miles to the east in
the mountains. While there, I noticed that high voltage transmission
lines (or, as they called there, "hydro lines") do not have ground
Everywhere else I have been, transmission lines usually have one or
two sets of three phases hanging on insulators plus an additional one
or two lines attached directly to the tower structure usually at the
highest point. I always assumed they were for lightning protection.
Wikipedia seems to support that. Anyone know why are they not needed
On Tuesday, June 23, 2015 at 5:20:55 PM UTC-4, Tony Hwang wrote:
You have a cite for that? From what I can see, looks like
they have some very limited installations where it's being
tested out and talk, but I don't see anything that indicates
power companies are sold on it, rapidly deploying it.
I'm not even sure what the exact point is. Guessing it's
reduced losses from skin effect?
And what's the relevance in terms of the question?
Typically here I see the smaller wires strung along the
tops of distribution towers like Pat says. Whether they
are there just for lightning protection or if they are
an essential part of the system, IDK. If it's 3 phase,
balanced, then no current would flow in them anyway.
On Tuesday, June 23, 2015 at 5:55:08 PM UTC-4, Ralph Mowery wrote:
I know it's not much, but those transmission conductors are big,
probably 1 1/4"+, so skin effect could have some economic effect
on the equation. And if it's not that, then IDK what the advantage
to going to DC is supposed to be? I can think of some big drawbacks,
like having to convert from AC to DC, then DC to AC again at those
huge power levels.
On Tuesday, June 23, 2015 at 6:45:49 PM UTC-4, Retired wrote:
Good find. So:
"HVDC requires less conductor per unit distance than an AC line, as there i
s no need to support three phases and there is no skin effect."
"It is because HVDC typically comprises active power flow only and thus cau
ses lower losses than HVAC lines, which comprise active and reactive power
"The disadvantages of HVDC are in conversion, switching, control, availabil
ity and maintenance.
HVDC is less reliable and has lower availability than alternating current (
AC) systems, mainly due to the extra conversion equipment. Single-pole syst
ems have availability of about 98.5%, with about a third of the downtime un
scheduled due to faults. Fault-tolerant bipole systems provide high availab
ility for 50% of the link capacity, but availability of the full capacity i
s about 97% to 98%.
The required converter stations are expensive and have limited overload cap
acity. At smaller transmission distances, the losses in the converter stati
ons may be bigger than in an AC transmission line for the same distance. Th
e cost of the converters may not be offset by reductions in line constructi
on cost and lower line loss."
Kind of what I thought, it's not exactly without issues of it's own.
One thing there that makes no sense is the part about it not requiring
3 phases. That's true, but so what? All those 3 phase conductors are
transferring power, just like all the conductors in a DC system would.
The reactive power part, I hadn't thought about, but that would be a
big thing, depending on how much power typically is reactive on a
power transmission line. You would think they would do everything they
can to get it close to zero.
One point made elsewhere in the article is that when DC is used to
interconnect 2 AC systems, they do not require synchronization of the
phases, or the frequency (50 vs 60 Hz).
Also, DC lends itself to underwater/underground HV cabling, where AC
Is there a valley to the left of the nearest toweer?
The local REAs have been using smaller wires and installing
them in twisted pairs. I'm guessing the twisted pairs shed
ice more readily than a single, larger conductor.
They also put what looks like a twisted rod in the middle
of some wire spans. Again, my guess is for ice/wind.
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